Capitalized Costs Relating to Oil and Gas Producing Activities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Supplemental Information on Oil and Gas Producing Activities [Abstract]
Proved properties	$ 240,217	$ 277,987	$ 293,486
Unproved properties	60,746	120,288	171,303
Wells, equipment and facilities	2,107,061	2,081,103	1,840,154
Support equipment	6,815	6,645	6,254
Capitalized Costs, Oil and Gas Producing Activities, Gross, Total	2,414,839	2,486,023	2,311,197
Accumulated depreciation and depletion	(693,123)	(710,948)	(609,380)
Net capitalized costs	$ 1,721,716	$ 1,775,075	$ 1,701,817